The present invention relates to semiconductor electronic devices, and more particularly, to semiconductor devices used as variable attenuation switched limiters and still more particularly to variable attenuation switched limiters implemeted with p-i-n diodes.
Microwave limiters are typically used in a microwave receiver as passive protection devices and are placed between an antenna and a mixer or low noise amplifier (LNA) to protect the mixer or LNA from burnout due to excessively large rf input from the antenna. The essential features of a limiter are small insertion losses at small-signal input levels. This feature preserves the receiver noise figure. Another feature is that the limiter has large insertion losses at large-signal input levels which also protect the mixer or LNA.
FIG. 1 is a block diagram of a typical microwave transmit/receive T/R system 2 with a limiter 1 located in the receive section 17 of the microwave transmit/receive T/R system 2 between an antenna transmit/receive T/R switch 9 and a low noise amplifier 7. A circulator 11 connects the antenna 3 to the antenna T/R switch 9 and also to the transmit section 19 and in particular to a power amplifier 13. A channel T/R switch 5 interfaces a phase shifter 4 t either the transmit section 19 or receive station 17.
During the transmit mode of operation (the channel T/R switch is connected to power amplifier 13 and the antenna T/R switch 9 is connected to impedance matching resistor 8), the low noise amplifier 7 is normally turned off to conserve power. The input VSWR (Voltage Standing Wave Ratio) of the low noise ampifier 7 is typically poor when the low noise amplifier 7 is turned off. One function of the antenna T/R switch 9 is to provide a good VSWR to the circulator 11. Therefore, during transmit the antenna T/R switch 9 is connected to the impedance matching resistor 8. The antenna T/R switch 9 has traditionally been implemented in one of two ways:
(a) through a hybrid p-i-n diode switch which uses shunt and/or series p-i-n diodes as the switching elements or
(b) through a monolithic Field Effect Transistor, FET, switch which uses shunt and/or series FET as the switching elements.
For both cases each path through the switch requires two series or shunt switching elements separated by a length of transmission line (typically but not necessarily a quarterwave length). The insertion loss of the switch is comprised of two components, the loss of the switching elements (and there are two devices contributing to this number) and the loss of the transmission line. At X-band frequencies, typical losses for hybrid and monolithic switches exceed 0.5 dB.
The system requirements of modern microwave transmit/receive systems require that the LNA have a minimum noise figure. The insertion loss (in dB) of the antenna T/R switch and limiter contribute directly to the noise figure of the LNA.
As depicted in the block diagram of FIG. 1 the antenna T/R switch 9 and the limiter 1 are two separate circuits which must be connected together by bondwires. This configuration results in high assembly cost and time.
Thus, prior art limiters and antenna T/R switches have problems including high insertion loss and high cost.